# CVE-2026-80856: fuse: fix invalidate lock leak on setattr writeback failure

> A local attacker can cause a denial of service by triggering a lock leak in the Linux kernel FUSE filesystem during specific file attribute changes.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 4 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-99419, BELL-CVE-2026-80856, DEBIAN-CVE-2026-80856, GHSA-4jx3-fh7f-pxhw, UBUNTU-CVE-2026-80856

## What is CVE-2026-80856?

CVE-2026-80856 is a denial of service vulnerability in Linux, published on 4 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

fuse: fix invalidate lock leak on setattr writeback failure

fuse_do_setattr() takes filemap_invalidate_lock() for a DAX truncate
(fault_blocked = true) and releases it at the out:/error: labels.  But
when a writeback flush is also needed, a write_inode_now() failure
returns directly and leaks the lock, so any later fault or truncate on
the file stalls on the stale rwsem.

For example, truncate(2) on a setuid file reaches fuse_do_setattr()
with both ATTR_SIZE and ATTR_MODE set:

  truncate(2)
  └─ do_truncate()
     ├─ dentry_needs_remove_privs()         # S_ISUID
     └─ notify_change()                     # KILL_SUID -> ATTR_MODE
        └─ fuse_setattr()                   # no killpriv:
           │                                #   ia_valid |= ATTR_MODE
           └─ fuse_do_setattr()
              ├─ filemap_invalidate_lock()  # IS_DAX && is_truncate
              └─ write_inode_now()          # is_wb && ATTR_MODE
                 └─ if (err)                # e.g. daemon -> -EIO
                    return err              # <- lock leaked

Fix this by adding an unlock label that releases the lock before
returning the error, and use it for the fuse_dax_break_layouts()
failure path as well.

## Which versions are affected by CVE-2026-80856?


## How do I fix CVE-2026-80856?

Apply the vendor's patch.

## Is CVE-2026-80856 being exploited?

There is no known exploitation of CVE-2026-80856 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80856 and fix it if it is affected.

What it is: CVE-2026-80856 is a denial of service vulnerability in Linux, published on 4 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-80856 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-80856/ (Markdown: https://insights.sbomify.com/CVE-2026-80856/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80856
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80856
- OSV: https://osv.dev/vulnerability/CVE-2026-80856

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80856/
Summaries are generated and may contain mistakes; the official description above is the record.